Time-varying environmental control of phytoplankton in a changing estuarine system.

Estuaries are among the most valuable aquatic systems by their services to human welfare. However, increasing human activities at the watershed along with the pressure of climate change are fostering the co-occurrence of multiple environmental drivers, and warn of potential negative impacts on estuaries resources. At present, no clear understanding of how coastal ecosystems will respond to the non-stationary effect of multiple drivers. Here we analysed the temporal interaction among multiple environmental drivers and their changing priority on shaping phytoplankton response in the Bahía Blanca Estuary, SW Atlantic Ocean. The interaction among environmental drivers and the number of significant direct and indirect effects on chlorophyll concentration increased over time in concurrence with enhanced anthropogenic stress, changing winter climate and wind patterns. Over the period 1978-1993, proximal variables such as nutrients, water temperature and salinity, showed a dominant effect on chlorophyll, whereas in more recent years (1993-2009) climate signals (SAM and ENSO) boosted indirect effects through its influence on precipitation, wind, water temperature and turbidity. Turbidity emerged as the dominant driver of chlorophyll while in recent years acted synergistically with the concentration of dissolved nitrogen. As a result, chlorophyll concentration showed a significant negative trend and a loss of seasonal peaks reflecting a pronounced reorganisation of the phytoplankton community. We stress the need to account for the changing priority of drivers to understand, and eventually forecast, biological responses under projected scenarios of global anthropogenic change.

Seasonal baseline of nutrients and stable isotopes in a saline lake of Argentina: biogeochemical processes and river runoff effects.

The seasonal variability of inorganic and organic nutrients and stable isotopes and their relations with plankton and environmental conditions were monitored in Lake Chasicó. Principal component analysis evidenced the strong influence of the river runoff on several biogeochemical variables. Silicate concentrations were controlled by diatom biomass and river discharge. Higher values of nitrate and soluble reactive phosphorus (SRP) indicated agricultural uses in the river basin. Elevated pH values (∼ 9) inhibiting nitrification in the lake explained partially the dominance of ammonium: ∼ 83 % of dissolved inorganic nitrogen (DIN). The low DIN/SRP ratio inferred nitrogen limitation, although the hypotheses of iron and CO2 limitation are relevant in alkaline lakes. Particulate organic matter (POM) and dissolved organic matter (DOM) were mainly of autochthonous origin. The main allochthonous input was imported by the river as POM owning to the arid conditions. Dissolved organic carbon was likely top-down regulated by the bacterioplankton grazer Brachionus plicatilis. The δ(13)C signature was a good indicator of primary production and its values were influenced probably by CO2 limitation. The δ(15)N did not evidence nitrogen fixation and suggested the effects of anthropogenic activities. The preservation of a good water quality in the lake is crucial for resource management.

Dissolved heavy metal(Cd, Pb, Cr, Ni) concentrations in surface water and porewater from Bahía Blanca estuary tidal flats.

The concentrations of dissolved cadmium, lead, chromium and níquel were determined in surface water column and pore water, collected from the extensive tidal plain at Bahía Blanca estuary, Argentina. Three different polluted areas were selected to study the spatial variation of these metals. The concentration ranges within the water column were slightly variable (Cd: 0.18-2.48 microg L(-1); Pb: 0.38-7.53 microg L(-1); Cr: 0.89-5.83 microg L(-1); Ni: 0.81-3.49 microg L(-1)), and displayed a clear gradient respect to the industrial area. Significant differences (contrast tests) between sites were detected for Cd, Pb and Ni. Very similar concentrations of Cd (0.18-3.41 microg L(-1)), Pb (0.38-5.83 microg L(-1)), Cr (0.89-9.37 microg L(-1)), Ni (0.81-6.56 microg L(-1)) were found in the porewater at all sites. The results suggested that both environmental compartments (water column and tidal flats porewater) may be affected by similar point and non-point heavy metal sources.